1. Field of the Invention
The present invention relates to a method for regulating a boost pressure of an internal combustion engine.
2. Description of the Related Art
In engines and in particular internal combustion engines, such as gasoline and diesel piston engines, the air charge in a combustion chamber of the engine is increased by the use of a compressor, such as an exhaust gas turbocharger, for increasing the performance. The pressure with which the air is compressed into the combustion chamber of the engine is also denoted as boost pressure and is generally measured by a pressure sensor in the vicinity of the combustion chamber. The pressure signal is fed to a closed loop system, which controls the exhaust gas turbocharger, thus setting a desired boost pressure.
Exhaust gas turbochargers, in particular, have a pronounced time constant, and thus react comparatively sluggishly to changed control signals, making it difficult to regulate the boost pressure. It is therefore advantageous if a direct state variable of the exhaust gas turbocharger to be regulated is determined. Particularly suitable for this purpose is the speed of the compressor of the exhaust gas turbocharger. Knowledge of the compressor speed is of particular interest, since during operation of the turbocharger, a certain maximum speed threshold may not be exceeded, since otherwise the turbocharger may be damaged due to exceeding critical stresses in the compressor wheel or a deformation of the compressor wheel, which results in the rotor scraping the housing.
The compressor speed is in principle calculable with the aid of a known compressor characteristics map, provided that certain variables such as, for example, the pressure upstream and downstream from the compressor, the air mass flow rate through the compressor and the temperature upstream from the compressor, are known. Based on these variables, the position of an operating point in the compressor characteristics map and thus the speed of the compressor are known, without a need for a sensor to be used for the speed determination.
In combustion engines, the maximum torque should be made available to the driver as early as possible. Accordingly, high boost pressures are required, in particular at low compressor speeds. However, high compressor speeds may result in the so-called compressor surge. The so-called compressor surge is a phenomenon which occurs inherently in turbomachines, when, at a given speed, the applied pressure ratio is too high. In this case, flow separations occur, which stimulate the compressor blades to vibrate, which in turn results in the destruction of the turbomachine. Operation in or above the surge limit must therefore be avoided in any case.
In the vehicle, the surge range is avoided by regulating the boost pressure. In combustion engines, compressor surge is, for example, avoided by the use of wastegate valves which divert a portion of the exhaust gas past the turbine, thus limiting the speed of the exhaust gas turbocharger, and accordingly limiting the compressor pressure. If exceeding the surge limit is to be avoided via a characteristic curve to which input data has already been provided, a safety margin must be maintained to avoid surge under all operating conditions, for example, operation at high altitude with reduced ambient pressure, interference in the flow by intake of impurities, clogged air filters, unfavorable operating points and the like, and under consideration of possible series deviation. This limits the operating range of the compressor.
The above-described surge detection from measured pressure fluctuations is an indirect detection of the surge phenomenon. Inversion of the air mass flow rate lowers the pressure in the volume. The intensity and speed of this reduction depends on the corresponding system and the placement of the pressure sensor and must therefore be reexamined and provided with new input data for each configuration. This again results in a non-negligible safety margin to the surge limit and additional data input complexity.
On the other hand, operation of the compressor close to the surge limit may be of interest, depending on the application. In combustion engines, as described above, the maximum torque should be made available to the driver as early as possible. Turbo compressors for fuel cell applications, however, have their maximum efficiency close to the surge limit, so that optimum fuel consumption may be achieved only when operating near the surge limit.
Published German patent application document DE 602 22 525 T2, for example, describes a method for controlling the boost pressure of a supercharged internal combustion engine, in which the surge limit is entered into a characteristics map, and a pressure ratio may be determined based on a parameter representative of the air flow rate through the compressor, and in this way it may be determined whether the surge limit is exceeded.
Despite the numerous advantages of the methods known from the related art for regulating a boost pressure of an engine, they therefore still have a potential for improvement.
Thus, in the last-mentioned related art, the preparation of the characteristics map for surge limit detection for compressors is complex, since it must be prepared by detecting the pressure and air mass flow rate value and comparing it against a surge characteristic detected in surge test benches using high-resolution pressure sensors. Alternatively, surge during operation is identified from measured pressure fluctuations.